The present invention relates to a process for producing crystals of salts of acidic amino acids and basic amino acids such as ornithine aspartate (referred to as OR.AS hereinafter), arginine aspartate (referred to as AR.AS hereinafter), and lysine glutamate (referred to as LY.GL hereinafter).
An equimolar mixture of an acidic amino acid and a basic amino acid has a high water solubility and thus crystals of a salt of an acidic amino acid and a basic amino acid cannot be obtained by the crystallization by concentration usually used for amino acids. In order to obtain crystals of such salts, a process in which methanol or the like is added to an aqueous solution of the salt to lower the solubility (normal crystallization procedure), a process in which an aqueous solution of the salt is added dropwise to a large amount of a solvent (reverse crystallization procedure), etc. have been so far employed.
In the normal crystallization procedure, precipitated particles tend to aggregate with one another owing to the low crystallinity of a salt of an acidic amino acid and a basic amino acid, and sometimes form a sticky mass to cause a failure in the separation of crystals. Therefore, the reverse crystallization procedure is generally employed. However, the product obtained by this procedure contain amorphous solid and it is pointed out that the resulting product has problems in quality such as contamination with other amino acids and the ash contained as impurities in the amino acids used as raw materials and the inequality in molarity of the two amino acids which form a salt. Further, there are also problems such as difficulty in the removal of a solvent in the drying step.